This invention relates to circuits using a transistor pair and more particularly to improvements designed to reduce current-gain modulation arising from differences in the collector-base voltages of the two transistors.
Some high precision circuits that use transistor pairs operating at high voltages often suffer from what is termed current-gain modulation. This is also known as Early effect. When the collector-base voltage of a transistor changes, for example, due to varying load conditions, the current gain, particularly the alpha, changes, as does the base emitter voltage that is required to produce a particular current. When two such transistors are operated in a current mirror configuration where the two load currents should always be equal and track each other, such operation is difficult to achieve, particularly where the two transistors operate with different base-collector voltages.